docs: show how to read collections in SSR mode

[jaredcwhite]

The SSR / Roda docs are currently lacking information about how to read collections in manually. For example, something like this in an initializer:

  init :ssr do
    setup -> site do
      site.collections.pages.read
      site.collections.some_other_collection.read
    end
  end

Collections aren't loaded by default for performance/memory reasons. We might want an omnibus "read all collections" option though just in case folks are willing to take the performance hit.

[jclusso]

I ran into this personally so I definitely think docs could be stronger here!